What does the coordinates node do?
You can try this kind of configuration :



The bounding box is considered as a local space from (0, 0, 0) to (1, 1, 1), and each vertex of your mesh has a position into this bounding box. The 'coordinates' node gives this position.

Here with a sphere, no vertex has coordinates greater than 0.8, so you don't have any white part (this is a top view) :